Seismic pressure transducers adapted for marshy terrains are known and widely used. One such pressure transducer unit comprises a hollow cylindrical perforated casing defining an inner cavity. An air-filled bag or bladder containing a pressure transducer element is mounted inside the cavity. The bag is made from a sound-transmitting material such as rubber. When the transducer unit is submerged in shallow water or marshy ground, the water fills the cavity through the perforations of the casing. Seismic pressure waves from the liquid are transmitted to the pressure transducer element through the wall of the bag and the air therein.
Such known seismic transducer units have certain drawbacks chief among which are: the wall of the rubber bladder has to be relatively thin in order not to unduly attenuate the arriving seismic pressure waves. But, a thin-walled bladder is susceptible of becoming easily damaged by sharp objects piercing through the perforations of the casing. When the bladder becomes ruptured, the air therefrom escapes, water enters the inner volume of the bladder and damages the transducer element. Another serious problem with such prior art transducer units is that mud enters the cavity of the casing and forms a "cake" around the bladder, thereby preventing effective acoustic coupling between the external fluid medium and the transducer element.
In U.S. Pat. No. 3,932,834, assigned to the same assignee, is shown an improved transducer unit which comprises a casing defining a cavity and having sound-transmitting windows for acoustically coupling the cavity with a fluid medium surrounding the casing. At least one pressure transducer element is mounted inside the cavity. A core completely fills the cavity and supports the transducer element in place. The core transmits the pressure wave energy from the fluid medium to the transducer element.
This invention is an improvement over the transducer unit described in said patent.